It is about trust and you have to be comfortable with the fact that the reservation remains in control if the owner. It can save you money and most rentals go off without a hitch, but ask a lot of questions and make sure you are clear with the owners rules so both of you are on the same page.
In the end, there is always a risk as the owner could cancel at any time. Going through a broker, while more, may give piece of mind as some hold back part of the fee until the renter checks in.
Good luck!